The present invention relates generally to the field of wireless Local Area Network (LAN) communications, and in particular to establishment and coordination of mobile terminal sleep phases within the LAN.
A new forthcoming standard for wireless LAN services having high throughput, ETSI HIPERLAN Type 2, promises to open new opportunities for both existing applications and new applications. Current versions and approved portions of the proposed ETSI HIPERLAN Type 2 standard are hereby incorporated by reference. HIPERLAN Type 2 LAN networks use a Time Division Duplex (TDD) airlink, meaning that an Access Point (AP) and a Mobile Terminal (MT) in the LAN network both use the same radio frequency to communicate with each other. The AP is connected to a Network (NW) such as an operator""s intranet, and the MT will in most cases be a wireless Network Interface Card (NIC) to a personal computer (PC).
FIG. 1 shows an example configuration for an exemplary HIPERLAN Type 2 system, including an AP 104 within a cell 102. MTs 106, 108 and 110 are also located within the cell 102. As shown in FIG. 1, the AP 104 can communicate via a wireless TDD airlink 112 with, for example, the MT 110. Within each cell, an AP for that cell selects the best frequency with which to communicate with one or more MTs within the cell. The AP""s frequency selection can be based on, for example, the AP""s measurements of interference at other frequencies, as well on measurements made by MTs within the cell.
In accordance with the proposed HIPERLAN Type 2 wireless LAN standard, a wireless LAN system includes a Medium Access Control (MAC) layer, which is implemented as a reservation-based MAC layer. FIG. 2 shows an exemplary MAC data frame 200 having an exemplary MAC frame structure, including a Broadcast Control Channel (BCCH) 202, a Frame Control Channel (FCCH) 204, a Downlink Channel (DLCHAN) 206, an Uplink Channel (ULCHAN) 208, and a Random Access Channel (RACH) 210. As shown in FIG. 2, the boundary between the DLCHAN 206 and the ULCHAN 208, as well as the boundary between the ULCHAN 208 and the RACH 210, can be changed in accordance with traffic requirements. Assuming that the MT 110 has been authenticated and a connection has been established between the MT 110 and the AP 104, then in order to send Uplink (UL) data via the AP 104, the MT 110 monitors the BCCH 202 and the FCCH 204 for the occurrence of random access opportunities. The MT 110 can then request uplink resources via the RACH 210, and the AP 104 will acknowledge the request for uplink resources and start scheduling UL resources in the TDD airlink 112 for use by the MT 110. In other words, when the MT 110 places a request for uplink resources, a reservation-based access starts.
When the AP 104 receives Downlink (DL) data from the network (NW) for the MT 110, the AP 104 either buffers the data and defers transmission of the data to the MT 110 if the MT 110 is sleeping, or transmits the DL data to the MT 110 at the next possible occasion. The AP 104 announces that it has data for the MT 110 (and/or other MT""s within the cell 102) by broadcasting a frame having the format of the frame 200, with a MAC-ID and a Data Link Control Channel ID (DLCC-ID) of the MT 110 in the FCCH 204 following the BCCH 202. In this situation, the FCCH 204 also contains the exact location of the data for the MT 110, in the DLCHAN 206 of the frame 200. An MT having a MAC-ID can have several DLCC-IDs.
Since MTs are often powered by finite sources such as batteries, the HIPERLAN Type 2 standard provides for a sleep mode for the MTs to conserve energy usage by the MTs. This sleep mode is outlined in FIG. 3. As shown in FIG. 3, at a first step 302, an MT sends a sleep request signal, which can include a suggestion by the MT as to how long the sleep interval should be, or in other words, the sleep duration, to an AP. The AP accepts the sleep request signal, decides the starting time and the sleep duration, and then in step 304 sends a sleep reservation signal to the MT indicating the starting time at which the MT should enter the sleep mode, and the sleep duration or time the MT should remain asleep before xe2x80x9cwakingxe2x80x9d to monitor the BCCH of a MAC frame from AP for the occurrence of DL data pending for the MT. The sleep duration can be, for example, an arbitrary number of MAC frames. At step 306 the MT enters the sleep mode, and then when the sleep duration expires at step 308, the MT awakens and monitors the BCCH for indications of DL data pending for the MT. If DL data is pending, the AP will notify the MT via the BCCH and schedule downloading of the DL data to the MT. An alternative is to poll the MT prior to scheduling data to avoid using unnecessary airlink resources, or, for robustness of the sleep concept, the AP can poll the MT prior to sending data to make sure that the MT has changed its sleep state and is prepared to receive data.
In particular, if the MT discerns that the BCCH contains a signal such as a pending data indicator, indicating that downlink data is pending at the AP for an as-yet undetermined MT, then the MT will analyze the content of a Slow Broadcast Channel (SBCH) in the MAC frame for a dedicated wakeup Protocol Data Unit (PDU) directed to the MT. The SBCH location in the MAC frame is given by an Information Element (IE) in the FCCH. In other words, the MT will check further to determine whether it is the MT (or one of the MTs) for which data is pending. If no downlink data is pending for any MT, then the MT returns to the sleep mode for another sleep duration time period, at the end of which it will awaken and repeat the cycle by monitoring the BCCH for a pending data indicator, etc. If no pending data indicator is present, or if the indicator indicates that no downlink data is pending, then the MT will go back to sleep.
FIG. 4 shows the case where an MT analyzes the SBCH in the MAC frame for a dedicated wakeup PDU. As shown in FIG. 4, when an MT sleep time expires at time 420, the MT first examines the BCCH 410 to determine whether the BCCH 410 contains a pending data indicator indicating that the MAC frame 406 contains data for an MT. The pending data indicator does not indicate which MT that the data, if present, is intended for. If a pending data indicator in the BCCH 410 does indicate that the MAC frame 406 contains data for an as yet unspecified MT, then the MT seeks to determine whether the MAC frame 406 contains data for it. It does so by analyzing the FCCH 412 for an indication as to where the SBCH 418 begins in the MAC frame. For example, the FCCH 412 can contain a predefined Information Element (IE) 414 that indicates where the SBCH 418 begins. For example, the predefined IE 414 can be defined to include a MAC-Identification (MAC-ID)=0 and a Downlink Control Channel Identification (DLCC-ID)=0.
The SBCH is located in the DLCHAN of the MAC frame 406. A DLCHAN can contain, or host, several logical channels, including the SBCH. These channels can include, for example, a User Data Channel (UDC), a DLC Control Channel (DLCH), where DLC stands for xe2x80x9cData Link Controlxe2x80x9d, a Dedicated Control Channel (DCCH), an In-Band Channel (IBCH), and the Slow Broadcast Channel (SBCH) mentioned above.
The MT then analyzes the SBCH 418 to determine if the SBCH 418 contains any wake-up PDUs that include the MT""s MAC-ID. If yes, then the MT knows that downlink data is pending for it, and the MT will stay active to receive the downlink data. If no, then the MT knows that no downlink data is pending for it, and it returns to the sleep mode automatically without announcement to the AP.
In a case where the MT has pending uplink data for transfer to the AP, then the MT can cut short its sleep duration timer or time period and request uplink resources from the AP by, for example, sending an uplink resource request signal on the RACH 210 of a MAC frame 200.
However, these methods suffer several drawbacks. First, during an interval between a first time when the MT analyzes the BCCH for the occurrence of a predefined PDU indicating a location of an SBCH in the MAC frame, and a second time when the MT waits for the SBCH to occur within the MAC frame to determine whether it contains a wake-up PDU including the MAC-ID of the MT, or a second time when the SBCH starts within the MAC frame, the MT has no interest in the MAC frame. Although the SBCH can be located right after the FCCH, which would minimize this time, the SBCH can also be located elsewhere in the MAC frame. Furthermore, there are typically different sleep levels, or levels of low power consumption, that the MT can achieve. In the lowest, both analog and digital portions of the MT are in a minimal power consumption mode. However, as those of ordinary skill in the art will recognize, when it is time to awaken the MT from such a deep sleep, the MT can require a comparatively significant amount of time to come awake. This is because, for example, a Voltage Controlled Oscillator (VCO) and a Phase Locked Loop (PLL) in the analog portions of the MT require time to synchronize. Thus, ideally the MT should revert to deep slumber for the interval between the first and second times, but if the interval is shorter than an amount of time necessary to put the MT into deep sleep and then reawaken it, then the MT cannot be placed into deep sleep during the interval. Thus, if the SBCH is not located immediately after the FCCH, then power requirements for the MT over the time of the MAC frame increase.
Furthermore, if the AP requests the MT to send an acknowledgment within the same MAC frame that it is awake after receiving and identifying a wake-up PDU in the SBCH, then in order to provide the MT with as much time as possible to respond to the wake-up PDU and prepare and send the acknowledgment, the SBCH should be located right after the FCCH. However, as indicated above, the SBCH can be located arbitrarily in the DLCHAN within the MAC frame.
In addition, in a situation where the AP organizes MTs within its cell into different sleep groups, each group waking up at a different time, then if a sleep group contains only one MT, then a necessary preamble in the SBCH is comparatively large and represents extra overhead as compared with sleep groups containing more MTs.
With respect to the sleep groups, for inactive MTs, or in other words MTs that are in a sleep mode without any data transmitted in uplink or downlink, the power consumption for the Wireless Local Area Network (WLAN) device at the MT includes power consumption during the time the MT monitors the BCCH, FCCH and SBCH of a MAC frame at the end of a sleep interval or duration, in order to detect the occurrence of downlink data pending for the MT, in addition to power consumption during the inactive periods. Since the AP decides when the sleep mode of an MT will start, the AP can allocate the MTs within its cell into different sleep groups. The groups can have the same or different tine durations, but awaken at different times so that they are not xe2x80x9cin phase.xe2x80x9d This can reduce power consumption at the MTs. For example, where downlink data is pending for only one MT in a group, if the group is small then fewer MTs must awaken to monitor the BCCH, FCCH and SBCH to determine whether the pending downlink data is for them, than if the group is large. Of course, smaller and therefore more numerous sleep groups require additional overhead and resources at the AP, so this is a tradeoff.
FIG. 6 illustrates the principle of organizing MTs into different sleep groups having different phases. As shown in FIG. 6, at time 610 a first MT (MT-A) sends a sleep request signal to the AP, including a proposed sleep interval or duration. At step 612, the AP sends a sleep reservation signal to the first MT, including a sleep duration and sleep start time that the AP has chosen for the first MT. The sleep start time can be in the form of an offset, for example a number of MAC frames after the current MAC frame, which the first MT should wait before entering the sleep mode. In step 614, a second MT (MT-B) sends a sleep request signal to the AP, including a proposed sleep interval or duration. At step 616, the AP sends a sleep reservation signal to the second MT, including a sleep duration and sleep start time that the AP has chosen for the second MT. The first MT then enters sleep in step 618 at the time specified by the AP, and then subsequently the second MT enters sleep in step 620 at the other time specified by the AP. The AP can select the sleep start times, for example, to add an MT to an already existing sleep group of MTs. At the step 622, the sleep duration of the first MT expires, and the first MT awakens to monitor the BCCH for an indication of pending downlink data and determine whether any pending downlink data is intended for it. If no downlink data is pending for the first MT, then it resumes sleep and restarts the sleep duration time at step 622, and then awakens upon expiration of the sleep duration at step 626 to start the cycle anew. At the step 624, at a time between the steps 622 and 626, the sleep duration for the second MT expires, and the second MT awakens to monitor the BCCH in the same manner as the described for the first MT, and proceeds in a similar fashion.
Although this method has some advantages, it also has specific features that can be disadvantages. For example, the resources necessary for the AP to optimally spread MTs among different sleep groups may require context information for each MT, buffer storage for each MT, and an awareness of when each MT""s sleep duration expires. For example, an expiration timer may need to be aware of each MT""s sleep phase ID or sleep group ID.
Accordingly, it is possible that the AP will divide its fleet of MTs into a smaller set of sleep groups or phases. Furthermore, most of the time when an MT belonging to a phase with other MTs wakes up in order to monitor the MAC frame for pending downlink data, the MT will discover that the downlink data is not intended for it but instead intended for another MT in the sleep phase or sleep group.
Since the SBCH does not have any predefined structure, the MT must analyze or monitor every PDU included in the SBCH. Furthermore, decoding failures that occur when an MT monitors the BCCH, FCCH and SBCH can cause problems for a sleeping MT because decoding failures can prevent the MT from becoming aware of downlink data that is pending for it. Depending on MT behavior, the effect can vary. For example, if the MT wakes up for every decode failure just in case downlink data might have been pending for it, then the MT will often awaken unnecessarily and thus consume extra power. It the MT ignores probable decode failures, then it may react slowly to receive downlink data pending for it. For example, it will sleep for an entire sleep duration before again checking for pending downlink data.
In Mobitex and pACT (Personal Air Communications System) systems, mobiles must know the concept of different sleep phases, which is not the case for HIPERLAN Type 2.
In accordance with an exemplary embodiment of the invention, problems such as those identified above are solved by using the FCCH channel in the MAC frame to convey wake-up announcements to one or more MTs. In accordance with a second exemplary embodiment of the invention, a wake-up announcement can instruct an MT to send a predetermined reply signal on an allocated uplink channel or on the RACH within the same MAC frame, so that the wake-up announcement functions as a polling request from the AP to the MT.
In accordance with another exemplary embodiment of the invention, a wake-up announcement for an MT can indicate that downlink data intended for the MT is contained later in the same MAC frame.
In accordance with another embodiment of the invention, a wake-up announcement for an MT can contain a null indicator, which indicates that the MAC frame does not contain downlink data for the MT and that the MT should not send a reply signal to the AP, but instead should remain awake to await downlink data that will be provided in the future.
In accordance with another exemplary embodiment of the invention, resources necessary to coordinate sleep groups having different time phases, for example AP resources necessary to handle phase IDs of different MTs, are conserved by using some or all of each MT""s MAC-ID. In accordance with an exemplary embodiment of the invention, power is further conserved by locating the information element (IE) in the FCCH that indicates where in the DLCHAN the SBCH starts, at the beginning of the FCCH so that an MT can go back to sleep during the remainder of the FCCH, to re-awaken when the SBCH starts. In accordance with an exemplary embodiment of the invention where wake-up PDUs such as wakeup IEs are located in the FCCH, the wake-up IEs are located at the beginning of the FCCH.
Where the MAC frame also includes an SBCH, the IE in the FCCH indicating where the SBCH is located in the DLCHAN can be located at the very beginning of the FCCH with the wake-up PDUs right after, so that after monitoring or analyzing the wake-up PDUs an MT will know if downlink data is pending for it, and when the SBCH will occur (in other words, where in the DLCHAN the SBCH is located).
In accordance with a another exemplary embodiment of the invention, the wake-up PDUs (in either the FCCH or the SBCH) are ordered in ascending or descending order by MAC-ID, so that an MT that is analyzing the PDUs can easily determine when remaining PDUs that it has not yet analyzed cannot contain its MAC-ID, and conserve power by cutting short its analysis and going to sleep early.
In accordance with another exemplary embodiment of the invention, the ordering of the PDU""s can alternate between ascending and descending, for example to ensure that different MTs are treated more equally.
In accordance with another exemplary embodiment of the invention, the AP can place non-wakeup PDUs before the wake-up PDUs (in either the FCCH or the SBCH) to ensure that the MTs analyze the non-wakeup PDUs. Alternatively, where the wakeup PDUs are located in the FCCH, an SBCH-IE (an IE indicating the location of the SBCH in the DLCHAN) can be located in the FCCH before the wakeup PDUs, so that all MTs, not just the ones for which there is a wakeup PDU, will monitor the contents of the SBCH.
In accordance with another exemplary embodiment of the invention, the wakeup PDUs (in the FCCH or the SBCH) can be ordered by MAC-ID, for example in ascending or descending order, so that when an MT encounters a decode failure but recovers during the sequence of wakeup PDUs, the MT can discern whether the portion of the sequence that it missed could have contained a wakeup PDU having its MAC-ID, and then act appropriately.
In accordance with another exemplary embodiment of the invention, where wakeup PDUs are located in the SBCH, an index can be provided in the SBCH prior to the wakeup PDUs. The index indicates where in the list of wakeup PDUs a wakeup PDU bearing an MTs particular MAC-ID might occur. For example, the index can indicate different ranges of MAC-IDs, so that an MT can go to sleep after analyzing the index and then awaken to receive the portion of the SBCH containing a possible range of wakeup PDU MAC-IDs that includes the MT""s MAC-ID.
In accordance with another exemplary embodiment of the invention, when an MT wakes up to monitor a MAC frame, if the MT finds an indication that data is pending, for example an indication in the BCCH of the MAC frame, then the MT will also monitor the next MAC frame for an indication that data is pending.